The present invention relates to a double floor structure comprising an upper floor supported by a support means provided on a lower floor.
As a general double floor structure, a double floor has been known in which a wiring space for signal cables and power supply cables for OA equipment and the like is provided between two floors, and a double floor structure for a clean room has been known in which an air conditioning chamber is provided under a lower floor and mounting of chips onto a printed substrate or the like is conducted. In these double floor structures, a plurality of round bar steel members having a diameter of about 9 mm and constituting post members at appropriate intervals are erected on the floor surface of the lower floor or on a floor ground member, floor member holding means are fixed to the round bar steel members at positions at a height of 300 to 600 mm from the lower floor, and a floor member is mounted astride the floor member holding means. In this case, the floor member is square in shape, and its four corners are supported by the floor member holding means.
In the mounting of chips onto the printed substrate, an operation of moving a bed with the substrate mounted thereon over a predetermined distance, positioning the bed, and mounting the chips onto the stopped substrate is repeatedly conducted by a mounting device at high speed. In this case, the chips are mounted onto the printed substrate at a pitch of several micrometers, so that a high positioning accuracy is required.
In some cases of the mounting device, the positioning of the substrate, namely, the movement and stoppage of the bed is repeated at a short period of 0.1 sec/cycle, and the bed with the substrate mounted thereon is moved at high speed at the period, so that the device itself constitutes a vibration source. In this case, in the conventional double floor structure, the post members are slender, and the floor member is supported at a high position of the post members, so that stiffness in the vertical direction and horizontal directions is low. Therefore, the vibration by the mounting device is largely amplified, and it takes a long time for the vibration to be damped, so that there is the problem in that the substrate positioning accuracy is lowered and the performance of the mounting device cannot be maintained.
On the other hand, examples of systems having a high stiffness and providing a high vibration-damping performance include a large type surface plate having a weight of about 3.5 to 5 t. The large type surface plate has a structure in which a stainless steel plate having an area of 1200 mm×3000 mm and a thickness of 60 mm, a plate member constructed by winding a stainless steel sheet around a precast concrete having a thickness of 200 mm, or the like is bolted to heavy steel members such as H-shaped steels. Since the surface plate is required to have high horizontalness accuracy at its surface, tremendous time is taken for securing not only the accuracy of the single body but also the horizontalness accuracy at the time of installation, so that the cost rises considerably.
Furthermore, large equipment is required at the time of installing the heavy large type surface plate, and the large equipment cannot be introduced where the installation site is a clean room. Therefore, it is necessary to work with special three-prongs and rollers, so that there is the problem of poor workability.